Bonding element having separate heating and agitating particles

ABSTRACT

A technique for bonding plastic materials includes placement of minute particles at the interface, either between the interface surfaces or within one or both of the interface materials. A particle-moving field, such as an alternating magnetic field where magnetic particles are employed, is created across the interface to directly and correspondingly impress the alternating magnetic field on the magnetic particles while the interface is in a generally movable or yielding state. The alternating magnetic field continuously moves the particles and by proper orientation causes them to oscillate, rotate or otherwise move within the fluid interface material to move and intermix the plastic material and thereby produce an improved interface bond between the materials. The magnetic particles may be selected to also respond to a radio-frequency magnetic field to rapidly generate heat and thus simultaneously create the fluid material at the interface with the material agitation. The magnetic particles may further include particles to reduce the reluctance of the flux path to increase the magnetic intensity.

This is a division of application Ser. No. 445,983, filed Feb. 26, 1974now U.S. Pat. No. 3,941,641.

BACKGROUND OF THE INVENTION

The present invention relates to an improved bonding method andapparatus and particularly to such a method and apparatus for fusing twoadjoining plastic surfaces.

Plastic materials have found widely varying applications both as basicstructural elements and as accessory elements such as coverings orenclosures of a rigid or a flexible plastic for packaging of otherelements and the like. Thermoplastic materials of similar as well asmany that have dissimilar characteristics can be permanently connectedthrough the heating of the interface to a critical temperature andapplying an appropriate pressure with the heat at the criticaltemperature. A particularly satisfactory method and system is shown inU.S. Pat. No. 3,574,031 to Heller et al. As more fully disclosedtherein, discrete susceptor particles are introduced into the fusionarea and subjected to a high-frequency magnetic induction field,generally in the radio-frequency range. The plastic material immediatelyadjacent to the particles is converted to a fluid state, either a liquidor at least a softened flowable state, as a result of the heat generatedin the particles as such. By the proper control of various parameters,the flowing condition of the plastic is accurately controlled andproduces a highly satisfactory fusion bond or weld at the interface ofthe surfaces. As further disclosed therein, the susceptor particles maybe disposed directly upon the surfaces to be joined or supported withinan intermediate bond agent or layer interposed between two opposedsubstrates. The particles may also be embedded directly within one orboth of the surfaces to be joined. As particularly disclosed in theabove patent and others held by present assignee, the use of thediscrete particles does produce a highly satisfactory and novel methodof heating and of sealing thermoplastic members. However, as alsopointed out in such art, the creation of an effective and firm bondbetween plastic materials requires consideration of many parametersincluding control of the proper temperature, proper pressures and, inparticular, control of the heat location. Further, the joining ofcertain different particular plastics is extremely difficult because ofthe different sealing characteristics -- thus, a particular optimumheat, pressure and time for the plastic of the one element may not bethe same for the plastic of the second element. Although an intermediatebonding agent may contribute to an improved bond because ofcharacteristics more compatible with the two different plastics, theresults have not been considered highly satisfactory in many cases.

SUMMARY OF THE PRESENT INVENTION

The present invention is particularly directed to an improved method andapparatus for bonding plastic materials and the like generally by suchfusion processes. Generally, in accordance with the present invention,the bonding surfaces are agitated or moved relative to each other toenhance the bonding action. In accordance with an important and uniqueconcept and teaching of the present invention, the bonding interfacesare agitated by a forced movement of minute interface particles as aresult of an impressed energy field with a corresponding movement of theplastic material to improve the interaction between the two surfaceswhile they are in the intermediate fluid or movable bonding condition.The agitation contributes to the intermixing at the interface withimproved wetting characteristic and increases the strength of the bondbetween the joining surfaces. In a particularly novel feature and aspectof the present invention, at least one of the interfaces is formed withmagnetic particles which are capable of forming magnets such for exampleas those disclosed in U.S. Pat. No. 3,665,856 which relates to anelectric field printing method or U.S. Pat. No. 3,526,708 which relatesto a magnetic printing method. In accordance with the teaching of thepresent invention, a magnetic field source means is applied to theinterface surfaces to directly and correspondingly impress a magneticfield on the magnetic particles while the interface is in the fluidjoining state. The magnetic field source means is in accordance with oneaspect of this invention operated to shift the line of direction of themagnetic field polarization relative to the bonding surfaces. Themagnetic particles, either as a result of premagnetization or inresponse to the impressed magnetic field, are individual magnets andwill tend to align themselves in a given direction within the impressedfield in accordance with the direction of polarization. Each time thefield direction changes, the particles tend to rotate or reorient in therequired direction in order to again align themselves in a correspondingrelative direction in the impressed field. Alternatively, if the bondingmaterial is an elastic nature, a pulse magnetic field may periodicallymove the particles with a stressing of the material which acts to returnthe particles between pulses. The moving impressed magnetic field willthus continuously move the particles and by proper orientation causethem to move back-and-forth within the fluid interface. The resultingparticle agitation will move and intermix the plastic material toproduce a desired increased interface bond strength. The agitationgenerating magnetic field is preferably a relatively low-frequencyalternating field impressed directly across the interface.

The present invention may with particular advantage employ a complexmagnetic field having the combination of an audio-or similarlow-frequency agitation field component and a radio-or similarhigh-frequency heat-generating field component. Thus, with magneticparticles of the character disclosed in U.S. Pat. No. 3,574,031 and thelike to form susceptor particles, the radio-frequency field componentwill interact thereon to rapidly generate heat and in a very controlledmanner within the interface area. The audio-frequency magnetic fieldcomponent will vary at a relatively slower rate and operate directly onthe magnetic particles as described above.

The shape of particles may affect the agitation and for improved resultsin the present invention may be formed as individual needle-like oracicular shaped elements. Such a particle will tend to align itself withthe principal or long axis perpendicular to the interface and inoscillating back-and-forth provide a high degree of agitation. Further,such needle-like or elongated shapes readily become magnetized to asemi-permanent magnet in the appropriate low-frequency magnetic field.

Further, the agitation of such particles should contribute to the fusionbonding not only in intermixing of the material but may also introduce amechanical interlocking of the material even though fusion may not becreated at a particular interface. This will further contribute to thebond strength at the interface, and is particularly desirable in thepresence of surfaces having different optimum bonding characteristics.

The particles may be a combination of different types to providedifferent functions for promoting of an improved bond. Thus, some of theparticles may have a characteristic to produce polarization foralignment with the low-frequency field and others may function as a heatsource as a result of the high-frequency field or impressed energy fieldwhile still further particles may be introduced to reduce the reluctanceof the flux path with an increased magnetic intensity at the interface.The single type particle producing the dual functioning of heat andagitation may be desirable in producing optimum heat location forpromoting a better flowable condition of the particles. The simultaneousimpressing of the relatively low-frequency mixing field andhigh-frequency heating field which produces the simultaneousinterrelated action would appear to provide maximum response andcharacteristic and is therefore particularly useful as the bondingmethod in high speed, commercial production processes.

In carrying out the present invention, either one or both of thesurfaces to be joined may be appropriately provided with the desiredmagnetic particles. Alternatively, a separate carrier agent or elementmay be provided and interposed between the surfaces which are to bejoined. This may be particularly desirable where the surfaces to bejoined have significantly incompatible bonding characteristic. Further,as a practical matter, the carrier bonding element can be readily massproduced as a rigid or flexible element which is readily applied as aninterface layer between other preformed elements to be bonded to eachother. The carrier bonding element may of course also be printed orotherwise coated on the substrate or surface, or dry powder and resinparticles may be applied by electrostatic coating techniques and thelike.

The present invention thus provides a highly improved bonding method andapparatus which can be readily applies to various structures, processesand devices. Although the several parameters normally carefully selectedand controlled remain highly significant, they do not have the samecritical significance in order to obtain a selected strength.

DESCRIPTION OF THE DRAWING

The drawing furnished herewith illustrates the best mode presentlycontemplated for carrying out the invention and described hereinafter.

In the drawing:

FIG. 1 is a diagrammatic illustration of apparatus illustrating themethod for bonding of a pair of superimposed plastic layer in accordancewith the present invention;

FIG. 2 is a substantially enlarged view of a portion of the stackedelements of FIG. 1 and illustrating one construction in accordance withthe teaching of the present invention;

FIGS. 3 through 8 are similar sequential views illustrating an idealcharacteristic interaction to be obtained with the method and apparatusof the present invention;

FIG. 9 illustrates an alternative apparatus and method without the useof an intermediate bonding element or layer for joining of a pair ofsurfaces.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring to the drawing and particularly to FIG. 1 the presentinvention is illustrated in an apparatus to form a connection between apair of overlapped substrates 1 and 2 which at least include a heatsoftenable material such as thermoplastic surfaces at the interface. Anintermediate bonding element 3 is interposed between the overlappinginterfaces to be joined and is especially constructed in accordance withthe teaching of the present invention of a plastic base 4 andinteracting particles 5 to provide a highly improved fusion bond to thetwo substrates 1 and 2 and thereby firmly connect the substrates to eachother. The substrates 1 and 2 are formed to a desired intermediate orfinal configuration; for example, strip elements having overlapped edgeswhich are to be interconnected. The bonding layer 3 is formed of aconfiguration and shaped correspondingly to the desired bonded interfacearea and is appropriately located between substrates 1 and 2. Thestacked elements 1 - 3, as diagrammatically illustrated in a simplifiedform in FIG. 1 and 2, are mounted within a heating and clamping assembly6 for joining of the substrates 1 and 2 by fusion bonding to the layer3. The structure illustrated is shown in simplified form for purposes ofclearly illustrating the significant aspects of the present invention,and as subsequently noted many various forms of apparatus may beemployed in a practical installation. For example, the substrates mightbe moving through the coil assembly with a pressure means to maintain aforce during the cooling of the bonded surfaces. The illustratedassembly 6 includes magnetic field generating means comprising upper andlower coil units 7 and 8 located to the opposite sides of thesuperimposed elements 1 - 3. The coil units 7 and 8 are shown wound onsuitable magnetic coil forms 9 and 10 and are connected to a suitableradio-frequency generating source 11 which preferably produces aradio-frequency energy. When employing particles such as disclosed inthe U.S. Pat. No. 3,574,031, the source preferably provides an output inthe megacycle frequency range which has of course been found to beparticularly operable. In addition, a second magnetic field meanssuperimposes a relatively low-frequency field on the high-frequencyfield. In the illustrated embodiment of the invention, the second fieldmeans includes a pair of magnetizing coil units 12 and 13 located withone coil unit 12 above and one coil unit 13 below the radio-frequencysource coil units 7 and 8. The coil units 12 and 13 are connected to asuitable relatively low-frequency generating source 14 producing energyin the audio-frequency range and which will typically operate in thefrequency range of 1,000 to 10,000 hertz. The simultaneous energizationof the coil units 7 and 8, 12 and 13 simultaneously generates andimpresses the two magnetic fields and particularly throughout thebonding interfaces defined by the intermediate bonding agent or element3. Although shown and described as coil units, any suitable transducermeans can of course be employed.

The coil units 12 and 13 may be wound on the common coil forms 9 and 10which are shown relatively movable to also apply bonding pressure alongthe bonding interface. The coil forms will normally be formed of a lowloss material to prevent heating and the like where the radio-frequencyheating energy is provided. Thus, although pole members might bedesirable to channel or concentrate the audio power, the radio-frequencyenergy would generally severely limit any useful or practical life ofknown coils. Further, although a dual coil construction provides coilsto opposite sides of the work area, a single coil system might also beemployed.

In accordance with the teaching of the present invention, theintermediate bonding agent or element 3 includes a thermoplastic carrieror base 4 which is compatible and fusible with the opposite substrates 1and 2 and will provide a reliable fusion bond therewith in the presenceof a proper heating of the interface. In accordance with the illustratedembodiment of the present invention the bonding agent 3 also includesthe plurality of distributed particles 5 which are especially selectedto respond to the high-frequency magnetic field to form heat centers orsources and to the relatively low-frequency magnetic field to formmagnetic elements which move into polarized alignment with the impressedrelatively low-frequency field. The particles may be prepolarized assmall permanent magnets or form semipermanent magnets as a result of theimpressed magnetic field.

More particularly, the radio-frequency field of coils 7 and 8 applied tothe magnetic particles generates heat at the interface providing thecontrolled transformation at the interface of the surfaces to a flowingor moving molten state thereby providing for the known fusion bondingphenomena. This result is fully set forth for example, in the previouslyreferred to U.S. Pat. No. 3,574,031, as a result of the hysteresis typeheat generation. However, when employing particles as a heat source, theusual eddy current phenomena may also be employed by selection ofappropriate conductive and generally larger particles. Theradio-frequency magnetic field, particularly when employing themegahertz range, changes so rapidly that the particles remainessentially fixed within the carrier.

The frequency of the second magnetic field is such that it will notgenerally produce significant heating, particularly with thenonconductive magnetic particles. The relatively lower frequencymagnetic field applied to the particles, however, results in movement ofsuch particles 5 to effect an agitation of the heated, flowableinterface material 1, 2 and 3 to thereby contribute to an effectivebonding thereat. Referring particularly to the enlarged sectionalfragmentary views of FIGS. 3 - 8, a single particle 5 is illustrated atthe interface of the substrate 1 and the bonding agent 3. For purposesof discussing the apparent functions of the described embodiment, FIGS.3 - 8 show an idealized condition for a single, needle-like particle 5located adjacent to and parallel with the interface at the initial pointor start of analysis. Assume that a particle 5 has been prepolarized orhad assumed the illustrated polarization with the north pole 15 to theright end and the south pole 16 to the left end of the illustratedparticle. When the polarity of the audio-frequency magnetic fieldproduces a south pole 17 to the top side of the assembly and a northpole 18 to the opposite side, the particle 5 will tend to rotate toalign itself with the impressed field as a result of the well-knownattraction of unlike magnetic poles and repulsion of like or the samemagnetic poles. As the field increases in this direction, the particle 5will continue to move or rotate and in an optimum state, rotates asshown in FIGS. 4 and 5 to align itself with the field. In so moving, theparticle 5 moves through the plastic material of elements 1 and agent 3.The softened material will, of course, flow about the particle 5.Depending upon the fluid state, and the rate of particle movement, aslight void or gap 19 may develop to the trailing face or edge of theparticle. When the energization of coils 12 and 13 reverses as a resultof the opposite half cycle output of the source 14 the impressed polesare reversed, as shown in FIGS. 6 - 8. The particle 5 will tend tocontinue to rotate in the same direction, or reverse rotation, as shownin FIGS. 6 - 8, to the initial position and therefrom to a reverseposition across the interface. The softened material of substrate 1 andagent 3 will, of course, flow about the particle 5. As the particle 5rotates, the softened material on the leading face of the particle isdriven or forced to move and portions of the two materials will tend tobe carried into the opposite element as shown in FIGS. 7 and 8 by thematerials 20 and 21. Upon another reversal of the audio-frequency fieldto the initial assumed polarity, the particle again rotates in the sameor opposite direction providing further movement and agitation of thematerial.

Thus the audio-frequency field will tend to continuously move oroscillate the particles 5, which will continuously agitate and intermixthe two materials thereby improving the wetting and bondingcharacteristics of the softened or fluid interface. The agitation shouldalso tend to move surface contaminents into the interior of thematerials and present cleaner surfaces at the interface to furtherpromote better joining of the materials. Further, the agitation may movethe particles from one substrate to the adjoining substrate. Theintermixed materials at the interface by appropriate agitation willoften permit development of a transition zone with a gradual transitionfrom the one substrate or material to the second substrate or materialwith an improved joining of the substrates. Thus, the improved wettingand mixing may even permit joining of plastics which have generally beenconsidered incompatible for normal fusion bonding.

Further, the magnetized particle action in carrying of the materialsalong the face or ends of the particle may produce a mechanicalinterlocking where the material continues as an extension from one layerto another as well as adding to the general fusion action; therebyfurther contributing to the bonding characteristic and strength. If theparticle movement is quite rapid, a void may develop behind theparticle. Generally, the magnetic particles will be appropriately wettedby the plastic material which might be provided by addition of chemicaladditions such as silane and voids should not be generated in anysignificant degree. Thus, voids, air pockets and the like foamy typeinterface structures will usually not be desired in order to produce amaximum bond. If the particle should terminate in a position such as inFIG. 7, the softened material behind the particle would tend to flow tofill the slight gaps or voids 19. This would also provide a mechanicaljoint between the materials of substrate 1 and agent 3 as at 22. Thecharacter of a void may, of course, vary and might be of an hourglassshape such that material flowing into the void would produce a tongueand groove connection. The illustrated interlocks have been shown invery simple form for purposes of explanation. In practice much morecomplex mechanical interlocks can be anticipated.

From a review of the idealized characteristic shown in FIGS. 3 - 8, thephysical configuration of the particles may affect the agitationcharacteristic. Smooth spherical members tend to oscillate within theplastic material with mimimum agitation whereas particles which areelongated needle-like members or elements will tend to produce greateragitation characteristics, and those of greatest length should havemaximum twisting torque. However, when employing the heating particlesas described above, the particle size is relatively small to prevent hotspots. Further, if the particles 5 have relatively roughened surfaces,such as shown in FIG. 2, the protrusions may further contribute to themovement of the softened material to produce a highly improved agitationand interlocking.

Generally, as noted previously, the radio-frequency field for generatingheat in the particles will be above 10,000 hertz and will normally be ofthe order of 100,000 hertz or above, and preferably substantially abovethat and generally in the megahertz range where the ferromagneticparticles of the previous Heller patent are used. The audio-frequencyrange for generating particle movement will typically be in a range ofabout 60 to 10,000 hertz in order to permit the desired movement of theparticles. The particles 5 in turn will be selected to produce optimumresults and may be of any desired construction and material which willrespond to the magnetic fields to produce the desired functioning. Therelationship between frequency range and particle would not appear to becritical.

One illustrative example of constructing the intermediate bondingelement or agent 3 would be milling approximately 20% by weight of gammairon oxide (Fe₂ O₃), magnetic iron (Fe₃ O₄) or like particles into thebonding carrier 4 while the latter is in a heat softened condition.Generally, useful heating has been obtained in the prior art byemploying between 2% and 50% by weight. Such forming processes have beenemployed in the addition of such particles to material for thehigh-frequency bonding concept. The longest dimension of the iron-oxideheat-generating particles 5, as in such art, is typically within therange of submicron to 20 microns. However, as previously noted,particles of magnetic metal alloy which generate heat as a result ofeddy currents may also be employed. Such particles may be severalhundred microns. The smaller nonconductive particles are desirable indistributing the heated areas completely over the interface and thusassuring a more complete interface bonding. The larger particles maytend to heat somewhat more quickly and as a result of the size providestronger localized mechanical forces. However, the large particlesmight, of course, also create localized rather than a continuousinterface bond which is promoted by the smaller nonconductive particles.

The intermediate bonding element 3 is particularly desirable in thoseapplications where it is desired to provide an intermediate layer forany reason such as simplification of the addition of the particles, orthe like. The bonding element 3 is also highly significant where thesubstrates have such different fusion characteristic that they will notreadily directly fuse to each other. The intermediate element 3 is thenformed with a carrier material 4 which does bond to each of thesubstrates and thereby produces a highly desirable connection betweenthe substrates.

For optimum results, pressure is preferably applied simultaneously withthe application of the magnetic fields to simultaneously produce thebonding pressure and the agitation. Improved results will, of course,also be obtained as the result of the agitated interface even though thebonding pressure is applied immediately following the application of themagnetic fields and while the intermixed material is in the molten orfusing state. For example, if a continuous bond is to be effected alongthe edges of elongated overlapped layers, the edges can be progressivelypassed through the magnetic field sources and then through suitablepressure applying devices such as a pair squeeze, rollers, not shown,located immediately downstream of the discharge end of thehigh-frequency magnetic field means. Further, the interface may bebrought to the appropriate movable state and then passed through anagitation field means. The in-line processing would be desirable inorder to permit separation of the field pole structure. However, in thesequential step processing of the bond areas, the relative minute heatgeneration particles and the rapid loss or transfer of heat to thesurrounding relatively cool material may call for only a short elapsedtime between steps.

Where the material of elements to be interconnected are readilycompatible and where one, or both of the elements, can be formeddirectly with the particles located in the interface area, theintermediate bonding element or layer 3 may, of course, be soeliminated; for example, such as shown in FIG. 9. The application ofFIG. 9 is similar to that disclosed in the copending application ofAlfred F. Leatherman entitled FABRICATING METHOD AND ARTICLE FORMEDTHEREBY, Ser. No. 363,177, filed May 22, 1973, where a plug type insert23 is to be secured within the end of a tube member 24. The small insertmember 23 is preformed with appropriate particles 25 dispersedthroughout the member 23. The preformed member 23 is located within theend of member 24. A coil unil 26 is located about the overlapped areaand an audio-frequency source 27 and a radio-frequency source 28 areconnected in series to the coil unit 26 to simultaneously impress therelatively high heating frequency field and the relatively low mixingfrequency field to the localized, overlapping area. The particles 25 atthe interface will function in the same manner as in the previous orfirst embodiment. The particles spaced from the interface will also tendto oscillate. This should not interfere significantly with the process.Thus, the radially outwardly spaced particles aligned with the coil unit26 may also cause a somewhat molten condition, but the material willreturn to the original state. The axially spaced particles will probablynot generate any significant heat and thus the particles will not moveas the resin remains in the solid state.

The illustrated embodiments have been described with each particlefunctioning both as a susceptor or heat source as well as a magneticallydriven agitator. The particles may be a mixture of different typesincluding particles some of which only function as susceptor particlesand others of which only function as an agitator source as well as stillothers which are dual function particles. In addition, where a magneticfield is employed, still other particles may be employed to concentratethe flux field and thus increase the field intensity at the interface.For example, a mixture of iron oxide particles, barium ferrite particlesand highly permeable iron particles would respectively provide high heatgeneration, good agitation and a low reluctance flux path. A mixturemight also be employed where a different type of energy based on, forexample, dielectric heating, was employed to activate the particles togenerate heat from that employed to activate the particles foragitation. Further, a direct heating means might be employed to createthe molten or flowing state at the interface, with a magnetic,electrical or other similar varying field applied to the particles tocreate the agitation. The common functioning particle should generally,however, provide optimum results if the particle material is selected toproperly respond to both the relatively high- and low-frequency fields,as the molten state is created immediately adjacent the particle andshould permit maximum agitation.

Further, although separate sources are shown, other means of creatingthe two fields might, of course, be employed. Thus, a basic audio fieldmay be established with a high-frequency field signal superimposedthereon. Further, the present invention employing the particles with anappropriate electric or magnetic agitating field provides a highlypractical concept of improving the bond characteristic. The improvementresults from the agitation or vibration of the interface materials andthus within the broadest aspect of the invention any method ofgenerating such vibration may be employed.

The present invention has also been particularly described in connectionwith thermoplastic materials, but can be also applied to thermosettingplastics. The several plastic materials to which the present applicationis applicable may be generically defined as thermoplastic materials.

The present invention thus provides an improved means of fusingadjoining plastic surfaces and like which can be temporarily placed in aflowable state or condition.

The invention has been particularly described with the use ofalternating current fields which have been widely used in the particleheat source field. However, pulse and transient fields such as shown inHeller U.S. Pat. No. 3,665,856 may be equally employed in creating thedesired agitating particle movement.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

We claim:
 1. In a bonding element having a surface for fusion bonding toanother surface, both of said surfaces being heat softenable, theimprovement comprising a plurality of different first and secondparticles dispersed throughout the heat softenable material of theelement, said first particles responding to a high-frequency magneticfield above the audio-frequency range to generate heat for raising ofthe surface to a bonding temperature and second particles different thansaid first particles and responding to a second magnetic field in theaudio-frequency range to move in response to said second magnetic fieldand thereby establish agitation of the surface at such bondingtemperature.
 2. The bonding element of claim 1 wherein said particlesinclude third particles defining a low reluctance magnetic path throughthe element.
 3. The bonding element of claim 1 wherein at least some ofsaid second particles are elongated needle-like elements, the surface ofsaid elements having a substantially roughened surface to define aplurality of sharp projections.
 4. The element of claim 1 wherein saidsecond particles are needle-like elements and form semipermanent magnetsin the presence of said second magnetic field.
 5. The element of claim 1wherein said heat softenable surface is a thermoplastic material, saidfirst particles being a nonconductive iron oxide and said secondparticles being a magnetic ferrite.
 6. The element of claim 5 andfurther including third particles including highly permeable ironparticles to produce a low reluctance flux path through the element.